Thermal inkjet hardcopy devices such as printers, graphics plotters, facsimile machines and copiers have gained wide acceptance. These hardcopy devices are described by W. J. Lloyd and H. T. Taub in "Ink Jet Devices," Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988). The basics of this technology are further disclosed in various articles in several editions of the Hewlett-Packard Journal [Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No.1 (February 1994)], incorporated herein by reference. Inkjet hardcopy devices produce high quality print, are compact and portable, and print quickly and quietly because only ink strikes the paper.
An inkjet printer forms a printed image by printing a pattern of individual dots at particular locations of an array defined for the printing medium. The locations are conveniently visualized as being small dots in a rectilinear array. The locations are sometimes "dot locations", "dot positions", or pixels". Thus, the printing operation can be viewed as the filling of a pattern of dot locations with dots of ink.
Inkjet hardcopy devices print dots by ejecting very small drops of ink onto the print medium and typically include a movable carriage that supports one or more printheads each having ink ejecting ink ejection elements. The carriage traverses over the surface of the print medium, and the ink ejection elements are controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to the pattern of pixels of the image being printed.
The typical inkjet printhead (i.e., the silicon substrate, structures built on the substrate, and connections to the substrate) uses liquid ink (i.e., dissolved colorants or pigments dispersed in a solvent). It has an array of precisely formed orifices or nozzles attached to a printhead substrate that incorporates an array of ink ejection chambers which receive liquid ink from the ink reservoir. Each chamber is located opposite the nozzle so ink can collect between it and the nozzle and has a firing resistor located in the chamber. The ejection of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the resistor elements. When electric printing pulses heat the inkjet firing chamber resistor, a small portion of the ink next to it vaporizes and ejects a drop of ink from the printhead. Properly arranged nozzles form a dot matrix pattern. Properly sequencing the operation of each nozzle causes characters or images to be printed upon the paper as the printhead moves past the paper.
In an inkjet printhead the ink is fed from an ink reservoir integral to the printhead or an "off-axis" ink reservoir which feeds ink to the printhead via tubes connecting the printhead and reservoir. Ink is then fed to the various vaporization chambers either through an elongated hole formed in the center of the bottom of the substrate, "center feed", or around the outer edges of the substrate, "edge feed."
The ink cartridge containing the ink ejection elements is moved repeatedly across the width of the medium to be printed upon. At each of a designated number of increments of this movement across the medium, each of the resistors is caused either to eject ink or to refrain from ejecting ink according to the program output of the controlling microprocessor. Each completed movement across the medium can print a swath approximately as wide as the number of nozzles arranged in a column of the ink cartridge multiplied times the distance between nozzle centers. After each such completed movement or swath the medium is moved forward the width of the swath, and the ink cartridge begins the next swath. By proper selection and timing of the signals, the desired print is obtained on the medium.
Thermal inkjet printheads require an electrical drive pulse from a printer in order to eject a drop of ink. The voltage amplitude, shape and width of the pulse affect the printhead's performance. It is desirable to operate the printhead using pulses that deliver a specified amount of energy. The energy delivered depends on the pulse characteristics (width, amplitude, shape), as well as the resistance of the printhead.
A thermal inkjet printhead requires a certain minimum energy to fire ink drops of the proper volume (herein called the turn-on energy). Turn-on energy can be different for different printhead designs, and in fact varies among different samples of a given printhead design as a result of manufacturing tolerances. These tolerances add to the uncertainty in knowing how much energy is being delivered to any given printhead. Therefore, it is necessary to deliver more energy to the average printhead than is required to fire it (called "over-energy") in order to allow for this uncertainty As a result, thermal inkjet printers are configured to provide a fixed ink firing energy that is greater than the expected lowest turn-on energy for the printhead cartridges it can accommodate. A consideration with utilizing a fixed ink firing energy is that firing energies excessively greater than the actual turn-on energy of a particular printhead cartridge result in a shorter operating lifetime for the heater resistors and degraded print quality.
Inkjet print cartridges can suffer from many sources of droplet ejection problems such as the formation of a viscous plug in the nozzle region resulting in a droplet that is difficult or impossible to eject, or formation of bubbles in the firing chamber that can cause misdirected ejection or no ejection at all. These problems can induce droplet trajectory errors, or can cause a nozzle to fail completely. These and other problems can occur when a particular nozzle has been inactive for some period of time when the printer is not in use. Also, when a page is printed not all of the nozzles on a print cartridge are necessarily used. The sensitivity of a particular inkjet system to these problems is highly dependent on the ink formulation, the geometry of the nozzle and firing chamber, and temperature.
Periodic "fly-by" spitting of the nozzles is a method for preventing or curing these reliability problems caused by nozzle inactivity. Spitting is the ejection of non-printing ink drops during printing operations and also spitting during routine servicing of the print cartridges. However, spitting in the spittoon causes a higher occurrence of aerosol due to the distance the droplet has to fly before it hits the absorber (less aerosol is generated while printing on media). Attempts in previous products have been made to minimize the distance from the printhead to the absorber. Some products have used what they call an `active chimney` which is a plastic wheel that is spit on instead of an absorber, the wheel is indexed and the dry inked scraped off into a `bucket.` These solutions address overall aerosol, which primarily results in cosmetic issues such as build-up on the inside of the printer. They do not provide adequate aerosol solutions in the case where there is a chemical reaction between ink and fixer in two or more adjacent print cartridges. When ink types between the different inkjet print cartridges are incompatible, additional failures can occur if the incompatible inks mix on the inkjet print cartridge nozzle member. This problem also occurs in ink systems which use one or more print cartridges containing a "fixer" solution which is designed to be chemically reactive with the inks so as to "fix" the inks on the media. This cross-contamination of inks, or fixer and ink, can occur during fly-by spitting due to aerosol drifting "downwind" and landing on the nozzle member of adjacent inkjet print cartridges. This cross-contamination of inks, or fixer and ink, on the nozzle member of inkjet print cartridges can cause nozzle blockage.
Accordingly, it would be advantageous to have a method to that reduces ink cross-contamination of print cartridges in inkjet printers during fly-by spitting.